The present invention relates to an apparatus for the manufacture of corrugated paperboard and, more particularly, to a heating apparatus for a double facer where a liner web is attached to a single face corrugated web and to a preheating device for a liner web or a single face web.
In a typical prior art double facer, a liner web is brought into contact with the glued flute tips of a single face corrugated web and the freshly glued double face web is passed over the surfaces of a number of serially arranged steam chests to cause the starch-based glue to set. Double face web travel over the steam chests is provided by a wide driven holddown belt in direct contact with the upper face of the corrugated web and the top face of the belt held in contact with the traveling web by a series of ballast rollers or the like, all in a well known manner.
Prior art steam chests, one example of which is shown in U.S. Pat. No. 3,175,300, are typically made of heavy cast iron construction in the manner of a pressure vessel in order to contain the high pressure steam which is supplied to the steam chest. For example, the walls of a cast iron steam chest are typically 1" or more thick to safely contain saturated steam supplied, for example, at 365.degree. F. and 165 psi (185.degree. C. and 1138 kPa). A steam chest has a flat upper web-supporting surface having a length in a transverse direction sufficient to support the full width of the traveling web and a width in the direction of web movement of typically about 18 inches to 24 inches (46 cm to 61 cm). Ten to twenty steam chests are typically serially arranged in closely spaced relation in a double facer.
The heavy cast iron construction of prior art steam chests results in a number of well known operational problems. The heavy walled construction of these steam chests requires a long time to bring them up to temperature on startup. Eventually, the steam chest may be brought close to the temperature of the steam being supplied to it. However, when operation is commenced and the double face corrugated web is traveling over the upper surfaces of the steam chests, heat is drawn therefrom at a rapid rate and surface temperature may drop to levels as low as 220.degree.-230.degree. F. This lower effective operating temperature may require the use of a substantially larger number of steam chests in a given double facer than would be necessary if more efficient heat transfer were attainable. The operating speed may also have to be reduced in order that the corrugated board may be properly cured. Another problem directly related to the inefficiency of heat transfer through a heavy iron steam chest casting is the transverse bowing of the upper surface of a conventional steam chest during operation. As indicated, the temperature of the flat upper wall of the steam chest is reduced substantially relative to the bottom wall of the steam chest resulting in a concave bowing of the upper surface lengthwise of the steam chest (transversely across the web traveling thereover). As a result, the holddown belt and transverse ballast rollers pushing the belt downwardly against the upper surface of the web do not impose a uniform load on the web. The result may be uneven curing of the adhesive, zones of poor or no adhesion, and crushing of the lateral edges of the web. Finally, the heavy mass of cast iron steam chests results in high heat retention and slow cool down, often requiring elaborate systems to lift the web or lower the steam chests to avoid excess heating of the web.
U.S. Pat. No. 5,183,525 includes a recognition of certain of the foregoing operational problems in systems utilizing heavy cast iron steam chests. In this patent, the steam chest is replaced by a heavy steel plate through which transverse horizontal bores are drilled and interconnected at their opposite lateral ends to form a serpentine steam passage through the plate. The holes may be drilled in a manner forming a much thinner web of material between the bores and the upper surface of the plate to increase the efficiency of heat transfer. The patent also teaches that the problem of bowing or distortion of the upper contacting face of the plate is minimized. However, the construction of the heating plates in this patent is still quite massive and heavy and, as is well known, the heat transfer efficiency of ferrous metals is relatively poor.
There remains a need, therefore, for a simple, efficient, and low cost hot plate system for a double facer which effectively addresses the problems typical of the prior art.
The web components of a double face corrugated web are also typically heated in the various stages of production of the corrugated board. The liner web and the medium web are typically preheated prior to their being joined together in the single facer apparatus. Similarly, the resultant single face web is also preheated prior to its being glued to the other liner web in the double facer. Preheating of the component webs is conventionally accomplished by causing the web to be wrapped around a portion of the circumference of a rotary preheating drum, the interior of which is heated with steam. Preheater drums are typically made of heavy walled cylindrical steel shells which, like conventional heavy walled steam chests described above, are slow to heat and slow to cool. Thus, variations in the amount of heat transferred to a web passing around the preheater drum is controlled with orbital wrap arms which can vary the amount of wrap which is applied to the web around the drum surface.
A description of conventional rotary drum preheaters and preheater control is contained in U.S. Pat. No. 3,981,758. In the manufacture of corrugated paperboard, however, board quality is typically best maintained if process variables, such as wrap arm adjustment on the preheaters, are minimized. Nevertheless, the amount of heat which is applied to the web components by the various preheaters must occasionally be varied and, because of the slow response time in heating or cooling the heavy metal preheater drum shells, wrap arm adjustment is the only practical means available to vary the amount of component web heating.
Therefore, it would also be desirable to improve the heating and cooling capabilities of prior art web preheaters, both to improve heat transfer response and to minimize the need for preheater wrap arm adjustments.